Water-absorbent resin exhibits high absorption rate, a large absorption amount, and a high retaining property with respect to an aqueous solution. Accordingly, for the purpose of absorbing body fluids, the water-absorbent resin has been conventionally used for absorbent cores of sanitary materials such as diapers, if necessary in combination with a fiber material made of cellulose fibers, polyester fibers, polyethylene fibers, or polypropylene fibers etc.
Recently, in response to increasing needs for thinner sanitary materials such as diapers, absorbent cores tend to include more amount of water-absorbent resin. In order to make the sanitary materials further thinner, it is required to replace the fiber materials with water-absorbent resin so as to further increase the amount of the water-absorbent resin in the absorbent cores.
The water-absorbent resin is excellent by nature in absorption and retention of an aqueous solution. However, the fiber material is not excellent in such absorption and retention, in particular not excellent in retention of an aqueous solution, and has abilities different from those of conventional water-absorbent resin. Therefore, in order to provide water-absorbent resin that meets the above requirement, it is necessary to develop water-absorbent resin with abilities of the fiber material included in a conventional absorbent core. Examples of abilities necessary for such water-absorbent resin include: an ability of diffusing an aqueous solution after absorbing the aqueous solution; and an ability of temporarily retaining an aqueous solution after absorbing the aqueous solution. That is, in a case where Centrifuge Retention Capacity (CRC; may be hereinafter abbreviated as “CRC”) that is an index for an ability of absorbing and retaining an aqueous solution is in a predetermined range lower than Centrifuge Retention Capacity (CRC) desired for conventional water-absorbent resin, in order that the water-absorbing agent absorbs an aqueous solution and then swiftly diffuses the aqueous solution in a perpendicular direction or a lateral direction, it is required to develop water-absorbent resin with high Saline Flow Conductivity (SFC; may be hereinafter abbreviated as “SFC”) that is an index for an ability of causing the aqueous solution to permeate in swollen gels under pressure.
An example of such water-absorbent resin is an aqueous solution-absorbing agent with CRC ranging from 5 to 25 g/g and SFC being 1216 cm3·s·10−7/g or more (see Patent Document 1 etc. for example). It is disclosed that such aqueous solution-absorbing agent can be obtained by polymerizing water-soluble ethylenically unsaturated monomer including a carboxyl group in the presence of an internal crosslinking agent including at least four functional groups each capable of forming a covalent bond with a carboxyl group.
Further, in order to provide a water-absorbing agent with increased liquid-permeability without dropping absorption capacity in a range of higher CRC, there is disclosed a water-absorbing agent that includes as a main component water-absorbent resin with a crosslinked structure obtained by polymerizing a carboxyl group-containing unsaturated monomer. The water-absorbing agent is internal crosslinked with non-polymeric compounds with at least four functional groups each capable of forming a covalent bond with a carboxyl group (see Patent Document 2 for example).
Although not regarding the water-absorbent resin with the abilities of a fiber material of an absorbent core, some documents report partially neutralized poly(acrylic acid) including polyethylene glycol. For example, regarding a uncrosslinked partially neutralized poly(acrylic acid) used in a sticking agent, it is reported that adding polyethylene glycol to a monomer aqueous solution containing partially neutralized acrylic acid and polymerizing the resultant results in uncrosslinked partially neutralized poly(acrylic acid) hydrogel that has excellent mold-releasability from a manufacture device such as a receptacle (see Patent Document 3 for example). Further, there is disclosed a method for performing aqueous solution polymerization on partially neutralized acrylic acid in the presence of polyethylene glycol with 400 molecular weight, in order to provide alkaline metal salts of crosslinked poly(acrylic acid) that are excellent in absorbency and viscosity-increasing property, that does not exhibit surface tackiness at a time of touching a liquid to be absorbed, that is adjusted to be in a safe range for human skin, and that is suitable for water-absorbent resin (see Patent Document 4 for example). Patent Document 4 describes that when water-soluble polyhydric alcohol such as polyethylene glycol is used as a crosslinking agent instead of a water-soluble polyfunctional vinyl monomer, a crosslinked structure can be formed by an esterification reaction between a hydroxyl group of the water-soluble polyhydric alcohol and carboxyl group. Furthermore, Patent Document 5 discloses a hydrophilic graft polymer capable of swelling and describes a graft polymerization between an acrylic acid and a polyalkylene oxide compound.
[Patent Document 1] International Publication No. 2007/037453, pamphlet
[Patent Document 2] International Publication No. 2007/037454, pamphlet
[Patent Document 3] Japanese Unexamined Patent Publication No. 2002-265511 (Tokukai 2002-265511; published on Sep. 18, 2002)
[Patent Document 4] Japanese Unexamined Patent Publication No. 1980-84304 (Tokukaisho 55-84304; published on Jun. 25, 1980)
[Patent Document 5] Japanese Unexamined Patent Publication No. 1993-239156 (Tokukaihei 5-239156; published on Sep. 17, 1993)